This invention relates to high voltage electrical switching apparatus and, more specifically, to high current, de-energized tap selecting switches used in power transformers. These transformers traditionally include tap changing mechanisms for allowing minor adjustments to the primary winding ratio. These adjustments are required to compensate for line voltage variations related to the physical distance from the point of power generation. The adjustments are generally made at the time of installation and remain unchanged as long as the power transformer remains at the sight.
The design of tap changing mechanisms is difficult due to the requirement for accurate contact positioning on all three phases of the transformer from a single manual operating mechanism located outside of the transformer. The switch contact surface area must be sufficient to prevent excessive temperature rise during all phases of operation with sufficient contact pressure to prevent arcing and/or welding during short circuit conditions.
The operating mechanism for tap changers of this type consisted of an externally mounted handle which would be rotated through a predetermined angular distance to obtain the desired tap setting. The problem with such an operating mechanism is largely due to the fact that the mechanism used to move the contacts to the desired position was also used externally to lock the operating handle, thus free play in the coupling apparatus could prevent full displacement of the tap changing switch even though externally the operating handle indicated full displacement, and was locked in position.
The contact design for this type of a switching apparatus must be capable of obtaining full current carrying capacity and at the same time maintaining sufficient contact pressure to withstand both normal and short circuit current stresses without welding or overheating. This has been generally achieved by a complex set of contact plates, springs, pivots and magnetic clamps which were used for the sliding bridge contact with cylindrical fixed contacts supported by insulating members. The problem with this arrangement is that only two lines of electrical contact can be guaranteed on each fixed contact regardless of size. Maximum current carrying capacity was thus limited to such line contact. In addition, the failure of a single spring could result in loss of contact pressure sufficient to cause failure of the switch as well as the transformer.
The fixed contacts in these systems were generally cylindrical in shape with a hollow end designed for the insertion of a stranded cable. The end was then crimped to secure the cable therein and to make a good electrical connection. This type of a contact is limited to one cable which severely limits the current carrying capacity in the system. The cabling had to be crimped into the contact prior to assembling the contact on the contact holder due to the limited clearance between the contacts. Because of the location of the switch, the cabling had to be bent to a right angle very close to the crimped area which often resulted in broken strands within the cable. This again limits or reduces the current carrying capacity of the contact and can produce high partial discharge levels resulting in dielectric failure. Precrimping of the cable assemblies requires extra cable length or where the cabling is short additional cabling length which would produce another potential problem area.